PROJECT SUMMARY Childhood obesity poses a critical public health concern, with obese children vulnerable to the development of potentially fatal chronic diseases. Scientific evidence has demonstrated that daily physical activity (PA) is imperative in preventing the onset of obesity. In an effort to develop engaging health interventions to promote PA among children, researchers are incorporating digital technology as a platform for health promotion. These interventions often focus on extrinsic rewards as the primary source of motivation, incentivizing children with points for being active. Emerging evidence indicates that technology-mediated health interventions that encourage children to focus solely on winning extrinsic rewards and isolate young people from social supports for PA have limited effects that may disappear altogether when the extrinsic rewards stop. We propose to evaluate a novel technology-mediated health intervention: the Virtual Fitness Buddy (VFB) Ecosystem was designed to encourage children to embrace PA as a lifestyle change rather than a means to gain points. The Ecosystem leverages existing social relationships in children?s lives, primarily the parent-child relationship, capitalizing on the power of social support that parents provide to children. Using consumer-grade technology devices, such as mobile phones, parents can stay involved in the intervention and support their children even when they cannot be with their children. In addition, a virtual pet, designed to mimic human-pet relationships in the physical world, will guide children to set and meet PA goals and offer tailored support. Using technology to amplify the power of existing social relationships, children will learn to embrace PA as a lifestyle change while supported by their parents and the virtual pet within the Ecosystem. In collaboration with YMCA, 720 children and 720 parents in 24 afterschool programs will randomly be assigned to treatment and control groups in a cluster-randomized controlled trial. Afterschool programs in the treatment group will offer the VFB Ecosystem intervention for a 3-month period; programs in the control group will receive standard of care without the Ecosystem. All children will complete baseline assessments, 3-month posttest, 6-month follow up, and a 12-month follow up of PA outcomes. Parents and youth will also provide self-report data on their perceived relationship quality, motivation, and PA self-efficacy. We aim to test the hypotheses that 1) youth assigned to the VFB Ecosystem will evince higher levels of moderate-to-vigorous intensity PA than those in the control group, and 2) test the parent and youth mechanisms of change, including parent-child interaction, children?s perceived level of social support, and parental involvement in children?s PA. The VFB Ecosystem represents a new generation of technology-mediated health interventions for children, which integrates technology into existing social systems, to promote sustainable lifestyle changes. Because the Ecosystem is a cost- and labor-effective solution that integrates consumer-grade technology with low barriers for continued use, it has the potential for rapid diffusion and widespread public health impact.